Although much has been learned over the years about the nature and function of the various components of the protein synthesizing system in mammalian cells, relatively little attention has been given to the molecular organization of these macromolecules within the cell. This facet of understanding protein synthesis is important in view of the relatively low efficiency of in vitro cell free systems. The specific objectives of this proposal are to examine the molecular associations among the various factors involved in protein synthesis in mammalian cells. Particular emphasis will be placed on the structure and function of a complex which contains all aminoacyl-tRNA synthetases, and on its role in the biosynthesis of proteins.